Generally, image matching is used to identify a person and letters based on image information. In image matching, a target image in which a target object is captured and a reference image in which the object has been captured and stored are compared for similarity so as to identify a person or letters contained in the captured target image. Images are compared for similarity generally according to SAD (Sum of Absolute Difference), SSD (Sum of Squared Difference), ZNCC (Zero-mean Normalized Cross-Correlation), or the like.
These techniques compare luminance values of a target image and a reference image. Thus, when images of an object are captured, if illumination conditions of the target image and the reference image differ, the luminance values of these images will vary and thereby the accuracy of matching tends to deteriorate.
Non-patent Literature 1 proposes a technique called increment sign correlation that performs robust image matching under illumination variation. The increment sign correlation method extracts only the sign of increment (relationship of magnitude) of luminance values of adjacent pixels in the horizontal direction for each of a target image and a reference image. Let f(x, y) as the luminance value of coordinates (x, y), an increment sign value h(x, y) is defined as Formula (1).
                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]                                                                      h          ⁡                      (                          x              ,              y                        )                          =                  {                                                    1                                                              (                                                            if                      ⁢                                                                                          ⁢                                              f                        ⁡                                                  (                                                                                    x                              +                              1                                                        ,                            y                                                    )                                                                                      ≥                                          f                      ⁡                                              (                                                  x                          ,                          y                                                )                                                                              )                                                                                    0                                                              (                  otherwise                  )                                                                                        (        1        )            
In the increment sign correlation, the number of pixels of which increment sign value of a target image matches that of a reference image is evaluated to match the images.
When illumination varies, the luminance value of each pixel and contrast (luminance difference) vary. Since the increment sign value represents the sign of the increment (relationship of magnitude) of luminance values of adjacent pixels, the increment sign value is hardly influenced by variation in the luminance value and contrast due to illumination variation.
Patent Literature 1 also proposes a technique that matches images for a target object using a plurality of reference images so as to improve the accuracy of matching. In Patent Literature 1, the increment sign values are obtained for each of a plurality of reference images and the probability of which the increment sign value becomes 1 is computed for each pixel. An image is generated based on computed probability values as pixel values and then the generated image and the target image are matched.
Patent Literature 2 presents a method that detects eyes from a facial image.